When analyzing signals, it is often important to compare signals one to another visually in order to detect meaningful differences. Since these differences are frequently subtle, it is at times difficult for technical workers as well as highly trained professionals, e.g., engineers, scientists, physicians, seismologists, economists, etc. to both learn the meanings of and properly interpret displays of data that they are considering. Exemplary of such situations are the multiple, time variance signals encountered in displays such as displays for non-destructive, eddy current testing; non-destructive ultrasound testing and acoustic emission testing; multiple trace EKG displays and displays for seismology, sonar and electro-encephalography (EEG).
Exemplary of an undertaking in which it is difficult to analyze signals without considerable know-how and experience is non-destructive, eddy current testing of objects such as tubes or plates. In eddy current testing, the impedance of an eddy current probe actually changes with the probe's position in a tube or position over a test object; therefore, an eddy current signal is actually a signal that varies with position. By moving the probe through a tube, over a tube or over a test object, this spatially variant signal becomes a time variant signal. Even experienced inspectors can easily make mistakes because displays of test signals are not necessarily clear enough for an inspector to analyze signals.
When inspecting tubes, such as the heat exchange tubes used in power plants, it is time consuming and frequently difficult for even experienced eddy current inspectors to distinguish between signal wave forms representing roll stops, through-wall holes, pits, and magnetic inclusions. Roll stops introduced during fabrication and magnetic inclusions are generally considered harmless, whereas through-wall holes and relatively deep pits are hazards. In prior art approaches, magnetic inclusions and pits produce substantially similar signals in which only an inspector's considerable experience can be trusted in making a decision as to whether or not a hazard exists in a length of tubing to the extent that the length of tubing should be replaced or removed from service by plugging both ends with a resulting loss of efficiency. Replacement of a length of tubing is a time-consuming, relatively expensive undertaking which should, of course, be avoided wherever possible. However, if the heat exchanger is used with a nuclear reactor then chances cannot be taken and therefore many tubes which may be sound are replaced or removed from service upon detection of an anomaly which it is suspected of being pit, but is merely a harmless magnetic inclusion (a magnetic inclusion is the occurrence of an iron particle or other magnetic alloy particle in the wall of a tube).
Generally, these signals are displayed as “figure 8” signals, known as lissajous figures. Lissajous figures are generated by an endpoint of a vector which represents an unbalanced voltage or impedance of a bridge and therefore, variations in voltage or impedance of detector windings of a probe. When a defect appears, a “figure 8” display occurs, with the peak-to-peak amplitude of the “figure 8” determining the volume of the defect and the phase corresponding to the depth of the defect. One way to recognize and differentiate between these defects is to apply signals of different frequencies. Typically, four frequencies are used and the resulting signals are displayed on four separate portions of a computer screen to the test object so that they may be visually compared. Visually comparing the signals in four separate sections of a screen is at times difficult so there is needed an approach in which the “figure 8” signals are visually displayed so that differences which relate to anomalies may be more readily detected and understood. That there is a difficulty involved in interpreting these signals is set forth in U.S. Pat. No. 4,763,274 issued Aug. 9, 1988, having the title “Machine Implemented Analysis Eddy Current Data”, incorporated herein in its entirety by reference. In an effort to make these signals easier to interpret, color displays have been used for strip charts as set forth in U.S. Pat. No. 4,644,336 issued Feb. 17, 1987 and titled “Color Display of Related Parameters”, incorporated herein in its entirety by reference. Color displays have also been used with lissajous figures as set forth in U.S. Pat. No. 4,631,533 issued Dec. 23, 1986 and titled “Display of Eddy Current Detector Data”, incorporated herein in its entirety by reference. These displays do not improve visual representation to an extent sufficient to reduce chances of error by either experienced or inexperienced tube inspectors, but serve only to indicate which signal on the screen pertains to a specific frequency channel. This can also be accomplished without the use of color by placing each eddy current signal at a different location on one screen and having some means to label each section of the screen to indicate the frequency channel.
Improving visual representation is a concern with other signals which are indicative of many different phenomenon. Exemplary of such are the frequency signals displayed for EKGs, seismology, sonar, EECs, music, other audible signals, and an entire host of situations where anomalies are uncovered by a comparative analysis.